46 Instinct and Intelligence 



when undisturbed can move from one to another 

 position; when any part of the surface of its 

 body or tentacles is mechanically or chemically 

 stimulated, it responds by contracting more or 

 less completely according to the intensity of the 

 stimulus. A localised stimulus applied at brief 

 intervals is at first followed by corresponding 

 contractions, but after a time the stimulus loses 

 its effect and contractions cease to occur; 

 though if the cause of irritation is continued 

 the Hydra loosens its foothold, and, if possible, 

 moves away from the cause which is irritating 

 it. Hydra only reacts to the stimulus of food 

 when hungry, and then the complex, and to 

 some extent co-ordinate movements of its 

 tentacles and mouth necessary to enable it to 

 seize, and swallow its food require a combina- 

 tion of mechanical and chemical stimuli to pro- 

 duce an efficient response of the structures 

 concerned in the action. 1 



The fact to which we desire to draw special 

 attention is that the most conspicuous move- 

 ments made by the animals referred to are pur- 



1 Am. Jour. PhysioL, Vol. VIII. p. 29, "The Animal Mind," 

 by M. F. Washburn, p. 214. 



